The production of a live program generally requires the use of a plurality of cameras that are connected to a so-called mixer. Such a mixer may have various levels of sophistication. The simplest type of a mixer is a switch, which selects one of the various cameras and provides its video signal at an output of the mixer. More sophisticated mixers may be able to superimpose images from two cameras so as to create a progressive transition or to provide a combined output signal in which one region of an image is supplied by one camera and another region by a second camera. In order for such a mixer to work, the video signals from the various cameras present at the inputs of the mixer must be perfectly synchronised. Such a synchronisation implies that the pixel frequencies of the various cameras should be identical, and that at the input of the mixer there should be no phase difference between the images from the various cameras.
Conventionally, each camera and the mixer have a time base of their own. If these time bases operate independently from one another, the pixel frequencies generated by these time bases will never be perfectly identical, no matter how high the level of precision of the time bases is. Phase differences between signals from various cameras may be caused by varying length of transmission line between the camera and the mixer.
In order to achieve a synchronisation in spite of these problems, two approaches have been developed. A first approach is the so-called image synchroniser. Essentially, an image synchroniser can be regarded as a type of adaptable buffer between each camera and the mixer. Image data are written into the buffer at the rate at which they are generated by the camera, which is based on the pixel frequency of the camera. They are read from the buffer at the pixel frequency of the mixer. If the pixel frequency of the mixer is exactly the same as that of the camera, the synchroniser can be seen as a simple delay element. If the pixel frequency of the mixer is higher than that of the camera, a frame stored in the buffer may be read twice by the mixer before it is overwritten by the camera, and if the pixel frequency of the camera is higher than that of the mixer, a frame in the buffer may be skipped (overwritten without having being read by the mixer at all). The rate at which frames are read twice or skipped is depending on the frequency difference between camera and mixer frequencies. The image synchroniser is versatile and easy to control, but it has a disadvantage in that it causes an unpredictable delay, which may take any value between 0 and 1 image. Such a delay can be extremely embarrassing when mixing video and audio signals to produce the program content. So called “lip sync” effects (audio signal leading or lagging video signal) are noticeable and unwanted effects.
This type of problem can be avoided using the genlock approach. According to this approach, the pixel frequency of each camera is controlled to be strictly identical to a pixel frequency of the mixer, and the image phase of each camera is controlled to have a slight advance over the image phase of the mixer, the advance being determined based on the delay of the transmission line between the camera and the mixer, so that when the video signal from that camera arrives at an input of the mixer, its image phase is strictly the same as that of the mixer.
A common approach to video device synchronisation can be described as follows. In a video system comprising a first video device which transmits a composite video signal comprising image information and synchronisation information derived from a time base of said first video device and a second video device which receives said composite video signal,    a) synchronisation information is extracted from the composite video signal received by the second video device and from the time base of the second video device;    b) horizontal and frame phase differences between the composite video signal received by the second video device and the time base of the second video device are determined based on said extracted H and F synchronisation information;    c) The horizontal phase difference is computed in the analogue domain, then applied to an integrator, resulting in a control voltage which will be sent on a line basis to the clock generation of the first device to shift up (or down) its frequency, reducing (or increasing) its line and frame duration until the horizontal phase difference is cancelled. This technique is well known as Phase Lock Loop.    d) The frame phase difference is processed in the following way: according to its value, a reset pulse is generated and transmitted on a frame or sub-frame basis to said first device to alter once only its regular (self running) period at a precise time.
Generally those two kinds of information (analogue proportional for frequency and horizontal phase control ; vertical reset for frame control) are combined to be sent to the first device.
It must be highlighted that the transmission media of the control information to the first video device must have predictable and reproducible delay. Otherwise, the reset pulse may be too soon or too late resulting in an erratic permanently wrong time base sequence.
Another specific example of a video camera system vertical reset processing is described in patent DE 40 30 148 C2. In this document, a frame phase difference between the sync pulses received from the camera and sync pulses generated by a local time base of the camera control unit is computed. If such a frame phase difference exists, a reset impulse is sent to the camera which will cause a phase shift of the image signal it generates (and, hence, of the V sync signal) which is equivalent to one image line. This process is repeated once per image (i.e. frame) until the control units finds that the V sync pulses from the camera and from the local time base are in phase. Since the number of lines in conventional video images according to e.g. PAL and SECAM standards is 625, this prior art system may need several seconds in order to bring the V sync pulses of the camera and of the control unit into alignment. Furthermore, it relies also on a perfectly predictable transmission delay of the reset pulse.
There is thus a need for a fast method for synchronising time bases of video devices and for a video device with fast synchronisation, notably when the devices are linked by 2 transmission media whose transmission delay may be varying over time.